1. Field of the Invention
The present invention relates to methods and apparatus for continuously determining the concentration of the constituents of the respiratory gases of a patient. Most preferably, the apparatus of the present invention is employed to monitor the end tidal CO.sub.2 and N.sub.2 O concentrations of a patient.
2. Description of the Prior Art
It is frequently of critical importance to monitor the concentration of carbon dioxide (CO.sub.2) in the gases inspired and expired from a patient under anesthesia, for expired CO.sub.2 concentration is a reliable indicator of the carbon dioxide concentration in the arterial blood. In a clinical setting, monitoring expired CO.sub.2 prevents malfunctions in anesthesia rebreathing apparatus from going undetected and delivering excessive amounts of CO.sub.2 to the patient. Rebreathing of anesthetic gases is very cost effective and environmentally desirable, but accurate CO.sub.2 concentrations are difficult to maintain in the patient circuit without a concentration monitor.
It is known that by directing infrared radiation through a sample of a gaseous mixture and measuring the incident radiation illuminating a detecting device, a measure of the infrared absorption of the gas can be obtained. Electrical signals produced by a detecting device in such a manner are thus indicative of the infrared absorption of the gas and can be processed to produce an output indicating the concentration of one or more of the constituents of the gas being analyzed. This type of gas analyzer operates on the principle that various gases exhibit substantially increased absorption characteristics at specific wavelengths in the infrared spectrum and that higher gas concentrations exhibit proportionally greater absorption.
Prior art infrared gas analyzers such as that described in U.S. Pat. No. 4,648,396 to Raemer utilize thermopile detectors to analyze gas concentrations. A thermopile detector is comprised of a number of thermocouples. Thermal detectors such as thermopiles are used primarily for infrared sensing and respond to the total incident energy illuminating them. The number of thermocouples must be sufficient to develop enough voltage to develop a suitable signal to noise ratio. A thermistor is used to calculate the Seebeck coefficient, which relates the voltage developed by the thermopile to the temperature differential between the "hot" and "cold" junctions of the thermopile. This coefficient is used to scale the output signal of the detector to indicate absolute gas concentration values.
Thermopile detectors as a class are known to suffer from thermal drift. Thermal drift causes a slow variation in the D.C. voltage output of the detectors and leads to measurement inaccuracies. Raemer overcomes this problem by using A.C. coupling between the thermopiles. This solution to the thermal drift problem, however, renders the resulting analyzer incapable of measuring absolute, steady state, gas concentrations. Instead, such devices are merely able to respond to changes in the concentrations of the gases being measured.
One method which attempts to obtain absolute gas concentrations involves modulating, or "chopping+, the incident energy beam. This technique is taught in U.S. Pat. No. 4,423,739 to Passaro. In order to accomplish the required modulation, mechanical means such as a chopper are frequently employed. The resulting analyzer is large, and is thus subject to failure as the moving parts of the beam modulation apparatus wear. Such failure may have catastrophic consequences rendering the analyzer inoperable.
An alternative method of modulating an energy beam is to construct a modulated source. Devices of this type are disclosed in U.S. Pat. No. 3,745,349 to Liston, for example. However, to date, analyzers employing modulated sources have suffered from slow response times due to the relatively long period required for the infrared emissions to decay after the excitation power is removed. Another shortcoming of modulated sources, familiar to those skilled in the art, is the relative instability of their energy output.
In addition, the imbalance between the excitation signals of two detectors has been utilized as a detection mechanism in the field of infrared motion detectors. In these devices, two detectors are arranged and their signals combined so that a null signal results when the detectors are identically excited. Any change in the environment that causes the output signal of one detector to change by an amount different than that of the other detector results in a non-zero differential signal. Thus, by subtracting the two output signals from two identical detectors an imbalance in the input excitation of the two detectors can be measured. An application of the method of subtracting two detector output signals, perhaps in an attempt to stabilize thermal drift, involves totally blocking one of the thermopile detectors with metal foil. This approach, however, is undesirable because of results in uneven heating of the substrate, thus causing an error signal to be introduced into the output.
Thus, at the present time, the devices available provide absolute concentrations with the trade-off of either decreased reliability in the case of mechanically shuttered devices, or slow response times in the case of modulated source devices. Eliminating either of these drawbacks using conventional thermopile detectors results in a device prone to inaccuracies due to thermal drift, or one incapable of determining absolute concentrations if the thermal drift is stabilized using A.C. techniques. Therefore, a reliable gas analyzer using thermopiles that is both immune to thermal drift and capable of providing absolute gas concentrations would be highly desirable.